This application relates generally to signal predistortion/postdistortion to compensate for undesired system distortion of a signal and in particular to closed-loop signal improvement by predistortion/postdistortion of a programmable converter.
In many communications applications, a signal is distorted by the hardware and channel over which the signal passes. An example where distortion occurs is a transmission system in a phone loop. Any signal, such as video, voice or data, passing through the phone loop, is subjected to the distortion introduced by the loop.
One prior art technique for eliminating noise in communications systems is taught in U.S. Pat. No. 5,251,328. The disclosed technique predistorts the amplitude of a transmitted signal in a manner which compensates for the amplitude distortion introduced in a portion of the communication channel.
Since a phone loop is formed by available equipment each time a call is placed, the distortion introduced will differ depending upon what equipment is utilized when the call is completed. Accordingly, since the distortion is not predetermined, compensation for distortion also cannot be predetermined.
It would be desirable to have a distortion compensation technique to ascertain the distortion introduced by a communications system so that information transmitted over the communication system, such as voice or data, could be predistorted/postdistorted such that the output signal would be distortion free.
In accordance with the present invention, a communications system transmits a predetermined signal, from a first, sending, end of a communications system to a second, receiving, end of a communications system. The predetermined signal is distorted by the communications system and received as a distorted signal that is compared to a known version of the originally transmitted signal. The difference between the originally transmitted signal (as represented by the known version of the originally transmitted signal) and the distorted predetermined signal as received at the receiving end, the transmitting end, or both, is employed to adjust a programmable converter at either the receiving end, the transmitting end, or both, to compensate for the distortion introduced by the communications system.